The present invention generally relates to a fault examining apparatus for a loop network system in which a master station is sequentially connected with a plurality of follower stations into a loop network using a single transmission path to transmit digital signals with serial information from the master station to the follower stations, from the follower stations to the follower stations or from the follower stations to the master station.
In recent years, as microprocessor technology has advanced, the functions and utility of automatic machines for industry use has advanced. It is not uncommon that the number of sensors and actuators used in such machines exceeds 100. Once automatic machines are lined within factories, mutual information exchanges and the information exchanges between computers for controlling production lines and the automatic machines are required to be effected without fail. In order to reduce the wirings within such facilities a described hereinabove or among such facilities, inputting, outputting control units (remote I/O) are installed within the facilities as the follower stations so as to connect with the follower stations the sensors, the actuators, and the signal lines of the communication information, while a connection is effected using only one optical fiber cable to a master station leading to the microprocessors for effecting the overall control operation so as to serially transfer the information.
FIG. 9 shows a system diagram having the remote I 10 units, wherein an input unit 3 and an output unit 4 are referred to hereinafter as follower stations, with sensors 1 and actuators 2 such as electromagnetic valves, and an optical fiber 6 being provided. A controller 5 for controlling all of the follower stations is called a master station.
FIG. 10 shows a schematic block diagram of the follower stations in the conventional embodiment. The control information from the master station is transferred to the follower stations as serial information composed of pulses as the intermittent signals of light. The transmission format of the serial information is composed of a start bit to allow a follower station to detect the head bit of the serial information, an address portion as the control information, a data portion, and a parity check bit for detecting the transmission error of the signals as shown in FIG. 11. In FIG. 10, reference numerals 7, 8, 9 show an optical fiber, an (optical-to-electrical) transducer, and an (electrical-to-optical) transducer, respectively. The input signals are connected to the sensors 1 of FIG. 9, while the output signals are connected to the actuators 2. All of the serial received signals RCV changed into electrical signals are once stored in a shift resister, wherein a serial.parallel conversion and a parallel.serial conversion may be effected. A circuit 15 checks whether or not the value of the address portion converted into a serial.parallel format conforms to the value of the address of the follower station established by the switch 16. When it is detected that the unit is accessed by an address conformity detecting circuit 14 after it is once stored in the shift register 14, the memory of the output data 11 of the shift register is effected by an output latch 17. The data portion of the serial information once stored within the shift register 14 is replaced into the input data 13 in accordance with the input signal so as to effect the parallel.serial conversion again for the transmission thereof as the transmission signal SND. When the address is not in conformity, it is transmitted through the parallel.serial conversion as it is. A timing pulse generating circuit 18 effects the controlling operation in the above-described procedure.
Only when through the above-described construction, the serial information transmitted from the master station of FIG. 9 is transmitted in order from the follower station to the follower station to come into conformity with the established address of the follower station, the input data of the follower station is sent into the optical fiber as the serial information and is returned into the master station. The master station decodes the address portion and the data portion of the returned serial information.
A transmission error detecting circuit 22 is adapted to detect the transmission error of the serial information. This circuit 22 detects whether the total sum of the bit numbers with the address portion and the data portion in the serial information being 1 is odd or even in number, and generates a transmission error signal if a predetermined parity error continues to be generated. In a gate circuit 23, the output of the latch pulse into the output latch 17 is stopped when the transmission error is generated and the output signal is retained in the previous condition. When the transmission error has been detected, the follower station transmits a received signal having an error as it is with respect to the next follower station. Accordingly, even in the master station, the signal returned by way of many follower stations is checked with the same circuit as the transmission error detecting circuit 22 being built in so as to detect that the transmission error has been generated within this loop.
It is to be noted that the main causes for generating transmission errors are a strong noise presence with respect to the electric circuits of the follower stations, a wiring condition of the optical fiber cables, changes in the bending condition of the optical cables with the cables being wired in a movable portion, and the received signal level of the optical signal of the follower station being beyond the proper range caused by an impropriety in the splicing portion of the light connector.
However, in above-described construction, if the master station can find the transmission error through a check of the returned signal, it cannot be easily checked how the level of the received signal under the conditions where the transmission error is likely to be generated is provided in an optional follower station. It is necessary to manually measure the level of the received signal in the follower station with a light-amount measuring instrument in order to have another look at the wiring of the optical fiber cable and the splicing portion of the optical connector when the level of the received signal at the follower station is excessively large or small. As a means for outputting the level of the received signal with respect to an optional follower station from the master station is not provided, the above-described operation is required to be effected one by one with respect to the follower stations in order to see which follower station within the loop is inferior in the wiring condition thereof, thus requiring increased time and labor consumption.